The present invention pertains generally to metal separation and in particular metal separation through photo-induced chemistry.
Present techniques for the separation of metals utilize a chemical difference in the metals. Examples of these techniques are fractional crystallization, ion-exchange chromatography, distillation, and solvent extraction. Generally, these techniques are slow and inefficient.
If the metals to be separated are lanthanides or actinides, the techniques are either unworkable or of very limited value. Only two techniques, ion-exchange chromatography and solvent extraction, are used commercially. Both methods are relatively slow and expensive, and require large capital expenditures for specialized equipment and large amounts of chemical reagents. The use of large amounts of reagents present serious disposal and storage problems, if the metals are radioactive, as in the case of nuclear fuel reprocessing which involves large amounts of lanthanides and actinides.
In light of these problems and disadvantages, metal separation by a photochemical method appears promising. To date, photochemical methods have been restricted to separation in the gas phase or in the solid state. Liquid-phase separation is considerably more difficult than separations in the gas phase or in the solid state. It is these problems which have discouraged research in liquid-phase, photochemical separation.
With liquid-phase separations, back reactions by radicals or other species in solution occur. For example, in reduction reaction in solution, the product is re-oxidized by radicals produced by the reaction. An important factor in controlling the undesired back-reaction is the speed of the reaction. Thus it is necessary for the reaction to go to completion much faster than any competing back-reaction. A liquid-phase separation requires a selective change in solubility in order to be successful. Often it is not possible to find a reaction which can produce a product which is sufficiently different in solubility.
Light-induced reactions have further problems, one of which is a lack of information concerning this type of reaction. Generally, light-induced reaction rates are too slow to avoid back-reaction to be practical. Often the wavelength of light needed to produce photo-oxidation also produces photo-reduction. Another difficulty is a lack of spectral selectivity of the reactants, e.g., metallic salts. The charge-transfer bands for most metal ions in solution are broad and overlapping.
There is considerable difficulty with chemical scrambling following photoredox processes, i.e., the product causing a further redox reaction with another species in solution. As a result, the first product is changed back to its original oxidation state and the wrong species changes oxidation states. In a separation process, the net effect of chemical scrambling is that the wrong species is separated or that no species is separated because the solubility of the species with the changed oxidation state did not change.
For those lanthanides and actinides displaying photoredox activity, there are generally only a few oxidation states available, and none of these elements can be photochemically reduced to the metallic state. This feature limits the choices for anions used to produce soluble salts in one oxidation state and insoluble salts in another. Furthermore, the oxidation potentials for a number of lanthanides and actinides are quite large (e.g., 1.55 V for Sm.sup.3+ /Sm.sup.2+), which increases the rate of back-reactions and side-reactions due to the greater reactivity of the photochemically produced oxidation state.